That’s a limitation with OSC and the UDP protocol. When a program starts listening or “binds” to a port, it becomes the only application that can listen to that port. Multiple applications can send OSC/UDP packets to that port at one time, but only one program can listen. The best workaround I can think of is having an application act as a repeater that can listen on one port and repeat that message out onto two different ports. I can’t think of any applications that can do this natively unless someone has written a specific tool. If you’re familiar with Processing.org, you could easily write a program that does this with the UDP examples. You wouldn’t have to decode the packet into OSC either, you could just forward UDP packets received to two different ports on the out.
There might be a way to do this with port forwarding built into your router, but I’m not as familiar with how that works. I’ll try some things and if I find an easy solution, I’ll post it back here.